The present invention relates to coatings to be applied to refractory metal cores to protect the cores from oxidizing during shellfire and from reaction/dissolution during the casting process.
Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components. The present invention will be described in respect to the production of superalloy castings, however it will be understood that the invention is not so limited.
Cores used in investment casting techniques are fabricated from ceramic materials which are fragile, especially the advanced cores used to fabricate small intricate cooling passages in advanced gas turbine engine hardware. These ceramic cores are prone to warpage and fracture during fabrication and during casting.
Conventional ceramic cores are produced by a molding process using a ceramic slurry and a shaped die. The pattern material is most commonly wax although plastics and organic compounds, such as urea, have also been employed. The shell mold is formed using a colloidal silica binder to bind together ceramic particles which may be alumina, silica, zirconia, and aluminum silicates.
The investment casting process used to produce a turbine blade, using a ceramic core is as follows. A ceramic core having the geometry desired for the internal cooling passages is placed in a metal die whose walls surround but are generally spaced away from the core. The die is filled with a disposable pattern material such as wax. The die is removed leaving the ceramic core embedded in a wax pattern. The outer shell mold is then formed about the wax pattern by dipping the pattern in a ceramic slurry and then applying larger, dry ceramic particles to the slurry. This process is termed stuccoing. The stuccoed wax pattern, containing the core is then dried and the stuccoing process repeated to provide the desired shell mold wall thickness. At this point, the mold is thoroughly dried to obtain green strength and the wax removed by application of high pressure steam which removes much of the wax from inside of the ceramic shell. The mold is then fired at high temperature to remove the remainder of the residual wax and to strengthen the ceramic material for the casting operation.
The result is a ceramic mold containing a ceramic core which in combination define a mold cavity. It will be understood that the exterior of the core defines the passageway to be formed in the casting and the interior of the shell mold defines the external dimensions of the superalloy casting to be made. The core and shell may also define other features such as core supports to stabilize the core or other gating which acts to channel metal into the cast component. Some of these features may not be a part of the finished cast part but are necessary for obtaining a good casting.
After removal of the wax, molten superalloy material is poured into the cavity defined by the shell mold and core assembly and solidified. The mold and core are then removed from the superalloy casting by a combination of mechanical and chemical means.
Attempts have been made to provide cores for investment casting which have improved mechanical properties, thinner thicknesses, improved resistance to thermal shock, and new geometries and features. One such attempt is shown in published U.S. Patent Application No. 2003/0075300, which is incorporated by reference herein. These efforts have been to provide ceramic cores with embedded refractory metal elements.
While it has been recognized that coatings are desirable to improve the performance of the refractory metal cores, there remains a need to define particularly useful coatings. Currently, chemical vapor deposition of aluminum oxide (alumina) is the baseline process/composition primarily due to availability and the excellent compatibility of alumina with molten nickel superalloys. A significant coefficient of thermal expansion (CTE) mismatch exists between the refractory metal/alumina that produces a microcracked coating. In its microcracked condition, the baseline coating is not entirely oxidation resistant during the investment shellfire.